The present invention relates to an air separation method and apparatus for producing a gaseous oxygen product at an above-atmospheric delivery pressure. More particularly, the present invention relates to such an air separation and method and apparatus in which the gaseous oxygen product is produced from a pumped liquid oxygen stream which is vaporized within a mixing column. Even more particularly, the present invention relates to such a method and apparatus in which air is separated within an air separation plant refrigerated by air expansion. Still even more particularly, the present invention relates to such an air separation method and apparatus in which the refrigeration is supplied from the mixing column to a lower pressure column of the air separation plant.
A variety of industrial processes require gaseous oxygen to be produced at an above-atmospheric delivery pressure. Such industrial processes include steel-making, glass-making and etc. Typically, air after having been filtered is compressed, purified and then cooled to a temperature suitable for its rectification by cryogenic distillation. The air is then introduced into an air separation unit that has higher and lower pressure columns connected to one another in a heat transfer relationship via a condenser/reboiler located within the lower pressure column. The air separates within the higher pressure column to produce a nitrogen-rich fraction and a liquid oxygen-enriched fraction, known as crude oxygen. The crude oxygen is further refined within the lower pressure column to produce a nitrogen tower overhead and a liquid oxygen column bottom. A stream of the liquid oxygen is pumped to the delivery pressure and then vaporized. The advantage of pumping is that expensive compressor units do not have to be used to pressurize the oxygen product stream.
Vaporization of the pumped liquid oxygen can be effected by direct heat exchange between the pumped liquid oxygen and a higher volatility stream within a mixing column. In the mixing column, the pumped liquid oxygen stream is introduced into a top region of the column and the higher volatility stream, which can simply be compressed air, is introduced into the bottom of the mixing column. The gaseous oxygen product is produced within the mixing column as a tower overhead.
In any air separation plant, there will be heat leakage into the plant through warm end losses and through heat leakage into the cold box. In order to compensate for this, refrigeration is added by way of expansion. In a common type of plant design, the incoming air stream, with or without compression, is either warmed or cooled to an intermediate temperature and is then expanded in an expansion machine with the performance of work to produce a refrigerant stream. The refrigerant stream is injected into the lower pressure column. This expanded gaseous stream, however, can have an adverse impact on the liquid to vapor ratio within the lower pressure column to decrease the production of liquid oxygen. This decrease in the production of liquid oxygen is reflected in a decrease in the production of the gaseous oxygen product.
As will be discussed, the present invention provides an air separation plant utilizing a mixing column to produce an oxygen product at an above-atmospheric pressure, air expansion to supply refrigeration, and a lower pressure column operating at an improved liquid to vapor ratio to increase oxygen production.